1. Field of the Invention
The present invention relates to a micro gas sensor and a method of manufacturing the same for precisely measuring a gas concentration, and more particularly, to a micro gas sensor and a method of manufacturing the same that has low power consumption, a rapid heating and cooling time, high durability, high sensitivity characteristics, a capability of easily forming a gas sensing film by using various materials, and a plurality of suspended sensing electrodes, which can be miniaturized and mass-produced at low cost through a simple manufacturing process using only a single pattern mask.
This work was supported by the IT R&D program of MIC/IITA [2006-S-007-02, Ubiquitous Health Monitoring Module and System Development].
2. Description of the Related Art
As concerns for the environment grow, the development of a micro sensor capable of obtaining precise and various information within a short time is required. Particularly, efforts to achieve miniaturization, high precision, and low cost of a gas sensor used for easily measuring a concentration of a gas have been continued to implement comfortableness of living spaces, cope with a bad industrial environment, and manage food manufacturing processes, etc.
Recently, a conventional gas sensor that has a ceramic sintering or thick film structure has been gradually developed to be a micro gas sensor having a microelectromechanical system (MEMS) type applying semiconductor manufacturing technologies.
In terms of measuring methods, in a method that has been widely used for the current gas sensor, when a gas is absorbed into a gas sensing film of a sensor, changes in electrical characteristics of the gas sensing film are measured. In this method, a metal oxide such as a tin oxide SnO2 is used as the gas sensing film, and changes in electrical conductivity due to a concentration of the gas are measured. This means the method is simple. Here, the metal oxide gas sensing film shows significant changes in measured values when the gas sensing film is heated at a high temperature and operated. Therefore, in order to rapidly and accurately measure the gas concentration, accurate temperature control is necessary. In addition, before the measurement, gas or moistures absorbed into the gas sensing film are compulsively removed by heating at a high temperature to recovery the gas sensing film to an initial state, and the gas concentration is measured. Therefore, temperature characteristics of the gas sensor directly affect main measurement factors such as measurement sensitivity, a recovery time, a response time, and the like of the sensor.
Therefore, for effective heating, a micro heater that can uniformly heat a local portion of the gas sensing film may be used. However, if a power consumed to control the temperature for measurement using the micro gas sensor is high, a large battery or power supply is needed although volumes of the sensor and a measurement circuit are small, and this causes an increase in a size of the entire measurement system. Therefore, in order to implement the micro gas sensor, a structure consuming low power has to be considered first.
Conventionally, in order to manufacture the micro gas sensor, a silicon substrate having a very high thermal conductivity is mainly used. Therefore, in order to reduce heat loss, etched pits or grooves are formed in a sensor structure by using a bulk micromachining process, a suspended structure separated from the substrate is formed, and a micro heater, an insulating layer, and a gas sensing film are sequentially formed on the structure, thereby reducing parts of the heat loss. However, in this case, wet etching using a crystallization direction of the substrate itself is used in the manufacturing processes, so that miniaturization of the sensor device is limited, and there is a problem in that properties of an etching reagent such as a potassium hydroxide (KOH) are not compatible with standard complementary metal-oxide semiconductor (CMOS) manufacturing processes.
In terms of commercialization, the micro gas sensor has to operate for two or three years stably. Considering that the gas sensor is repeatedly heated or cooled by the micro heater at a temperature ranging from 150° C. to 500° C., that is a very strict condition. Here, failures may occur due to mechanical impacts and thermal stress caused by a temperature gradient applied to the sensor structure suspended from the substrate. Therefore, in preparation for the failures, in a conventional method, a basal supporting layer of the suspended structure is formed by a single layer including a silicon dioxide SiO2 layer or a silicon nitride Si3N4 layer, stacked layers, or multi-layers thereof to be Si3N4/SiO2, SiO2/Si3N4, Si3N4/SiO2/Si3N4, or SiO2/Si3N4/SiO2 to implement stress balance. Thereafter, the micro heater, the sensing electrode, and the gas sensing film are sequentially patterned to solve vulnerability in the structure. However, although the stress balance is implemented, the aforementioned insulating materials are fragile and have low thermal conductivity. Therefore, if lacks of temperature uniformity increase in the heated suspended structure, structural safety cannot be guaranteed. Therefore, durability of the micro gas sensor device is related to materials of the structure and design of the micro heater.
In addition, to form functional elements including a thermal insulation structure for the conventional micro gas sensor device, by using five or six, or ten or more pattern masks, on a substrate, thin film deposition, photoresist coating, micro-patterning, thin film etching are repeatedly performed to sequentially stack and manufacture the functional elements in a vertical direction. Therefore, there are problems in that manufacturing cost consumed for the semiconductor manufacturing processes are increased, it takes a long time, and a yield is decreased.
Recently, for a gas sensing film having a core role in the micro gas sensor, materials including metal oxide, metal, semiconductors, polymer, nano-materials, and the like have been studied. A conventional gas sensing film is formed by directly micro-patterning sensing film materials on a surface of the sensor device by using a lift-off process using a pattern mask, or formed by moving the sensor device mounted to a micro stage to a lower portion of a micropipette to be micro-aligned, titrating materials, and performing post-treatment thereon. However, the aforementioned methods have a problem in that a unique micro-patterning process according to materials to be used for the gas sensing film has to be developed. Particularly, when the nano-materials are used, the problem is worsened. In addition, in a case where a high-cost precise mechanical micro-alignment apparatus is needed, additional manufacturing cost and time are consumed. Therefore, a technique for easily forming a micro pattern on a particular position of a surface of the micro gas sensor device so that various types of material can be used for the gas sensing film is needed.